Bitcoin uses Bitcoin Improvement Proposals (BIPs) as a design document for introducing new features or behaviour into bitcoin. Bitcoin Magazine describes what a BIP is in their article What Is A Bitcoin Improvement Proposal (BIP), specifically highlighting how BIPs are not necessarily binding documents required to achieve consensus.

The BIPs are currently hosted on GitHub in the bitcoin/bips repo.

BIP process

The BIPs include BIP 2 which self-describes the BIP process in more detail. Of particular interest might be the sections BIP Types and BIP Workflow.

What does having a BIP number assigned to an idea mean

Bitcoin Core issue #22665 described how BIP125 was not being strictly adhered to by Bitcoin Core. This raised discussion amongst developers about whether the code (i.e. "the implementation") or the BIP itself should act as the specification, with most developers expressing that they felt that "the code was the spec" and any BIP generated was merely a design document to aid with re-implementation by others, and should be corrected if necessary.

This view was not completely unanimous in the community.

For consensus-critical code most Bitcoin Core Developers consider "the code is the spec" to be the ultimate source of truth, which is one of the reasons that recommending running other full node implementations can be so difficult. A knock-on effect of this was that there were calls for review on BIP2 itself, with respect to how BIPs should be updated/amended. Newly-appointed BIP maintainer Karl-Johan Alm (a.k.a. kallewoof) posted his thoughts on this to the bitcoin-dev mailing list.

In summary a BIP represents a design document which should assist others in implementing a specific feature in a compatible way. These features are optional to usage of Bitcoin, and therefore implementation of BIPs are not required to use Bitcoin, only to remain compatible. Simply being assigned a BIP does not mean that an idea is endorsed as being a "good" idea, only that it is fully-specified in a way that others could use to re-implement. Many ideas are assigned a BIP and then never implemented or used on the wider network.